Try this:
set your computer (or the whole network, by doing this in the router settings) to use either 1.1.1.1 or 8.8.8.8 as the primary DNS server.
Stop Plex server
Enter the Plex data folder, then the subfolder Cache and remove the file cert-v2.p12.
Use the .reg file from You need to reclaim your server, but don't dare to touch the Windows Registry
Then claim the server as explained in New Claiming Requirement for Windows
